The present invention relates generally to complementary symmetry amplifier circuits and more particularly to a biasing arrangement for a complementary symmetry amplifier.
In complementary symmetry amplifier circuits such as that generally found in the vertical sweep output circuit of a television receiver, two active devices, i.e., transistors of opposite conductivity are arranged to operate as emitter followers for delivering output current to a load (the vertical deflection windings). The transistors are biased so that approximately one half the available supply voltage appears across the collector-emitter junction of each device. The emitter terminals of the transistors are therefore nominally biased to approximately one half the supply voltage. The base terminal of each transistor is connected such that, for an input signal more positive than one half the supply voltage, one of the transistors supplies load current, and for an input signal more negative than one half the supply voltage, the other of the transistors supplies load current, i.e., only one transistor at a time supplies load current.
A problem associated with complementary symmetry amplifiers is that the base-emitter diode voltage drop of each transistor must be overcome before the transistor can conduct. In silicon transistors of the type commonly used, approximately 0.7 volt is required to overcome the base-emitter voltage drop. As the input signal voltage changes from a level above one half the supply voltage to a level below one half the supply voltage, there is a narrow range over which neither of the output transistors is conducting. This results in a flat spot on the transistor transfer characteristic which adversely affects the amplifier output and creates a problem known as crossover distortion. In a television receiver, such crossover distortion in the vertical sweep output amplifier is visibly noticeable as a white line across the face of the picture tube.
Various circuits have been devised heretofore to avoid the problem of crossover distortion. Noteably among these are feedback arrangements and numerous voltage divider networks for developing the necessary 0.7 volt to bias each of the transistors slightly into conduction. The feedback methods of preventing crossover distortion, which require a feedback network, have proven unacceptable because of the frequency requirements of the feedback network. Most of the distortion correction schemes have therefore utilized various methods of biasing the two complementary transistors. In general, these biasing techniques utilize a plurality of resistors or a combination of diodes and resistors to cause a small bias current to flow into the base of one transistor and out through the base of the other transistor. This type of circuit causes a quiescent current to flow through the collector-emitter path of the two transistors. The foregoing type of circuit suffers from the likelihood of thermal runaway since any increase in temperature of the active devices causes the quiescent current to increase thereby further increasing the temperature within the devices and again increasing the quiescent current. Prior art attempts have been made to overcome this thermal runaway problem by carefully matching the complementary transistors with diodes and/or biasing transistors such that the changes in the temperature of the output devices is compensated for by changes in the temperature of the biasing devices. This technique has been only partially successful since it requires that the biasing devices and the output devices be mounted on a common heat sink and that they be carefully matched one to the other. Failure of any one of the devices therefore requires that extreme care be taken in either the selection of a replacement device or requires so that all of the devices be replaced with a matched set in order to prevent thermal runaway and crossover distortion.
Accordingly, it is an object of the present invention to provide a biasing arrangement for a complementary symmetry transistor amplifier which is thermally stable and substantially reduces crossover distortion.
It is a further object of the present invention to provide a biasing arrangement of a complementary symmetry transistor amplifier in which the active devices may be discretely replaced without regard to matching of components.
Another object of the present invention is to provide a biasing arrangement for a complementary symmetry transistor amplifier in which a common heat sink is not required to provide thermal stability.
In accordance with the present invention, in a complementary symmetry amplifier utilizing transistors of opposite conductivity for output devices wherein the emitter electrodes of the transistors are interconnected to form the common output terminal and the collector electrodes are coupled across a power supply, there is provided a biasing circuit comprising a control transistor and a resistor network coupled in parallel with the control transistor to provide bias voltage to the base electrodes of the amplifier transistors. The control transistor has its base electrode connected to the base of one of the amplifier transistors and an output electrode connected to the base of the other of the amplifier transistors. The resistor network is arranged such that a first element is connected between the emitter and collector terminals of the control transistor. Input signals are applied to one terminal of the control transistor and a biasing current is supplied from a constant current source to the resistor network to develop a voltage across the first resistor element. This bias voltage is just sufficient to turn on the two complementary output transistors. The base current which is drawn by the first output transistor also flows through the base circuit of the control transistor thereby effecting the conduction of the control transistor. Increased conduction of the control transistor reduces the bias voltage appearing across the first resistor element which in turn reduces the bias for the two output transistors. A quiescent level of output current is accordingly reached which depends upon the parameters of the three transistors.